I have posted this as an ACR and Lightroom Feature Request at the Photoshop Family site.
It was suggested I also post it here to for more visibility.
"The current Histogram display in Lightroom and Adobe Camera Raw accurately display the file image data for JPEG, PNG, PSD, TIFF file types, but not for raw files.
When a raw file is demosaiced and a camera profile applied it is no longer possible to determine the actual R-G-B-G2 raw data clipping levels. This is useful for determining best exposure settings (ETTR) when shooting tethered, evaluating exposure bracketed images, or for simply determining your camera's exposure metering accuracy.
This can be added as a user-selectable option button in the current Histogram to allow viewing of the camera file's R-G-B-G2 raw data values."
This has been requested in the past, but there appears to be mixed feelings and some confusion as to its benefits:
The sole objective of using the raw image file data in the Histogram is to determine if the image file contains clipped pixels. The actual raw data preview image is of no value other than to "accurately" display those pixels using the Highlight and Shadow clipping indicators. A good example of this implementation is the RawDigger application available for download here: